This invention in general relates to interferometry and in particular to interferometric apparatus and methods by which the local surface characteristics of photolithographic stage mirrors or the like may be interferometrically measured in situ or off stage to provide correction signals for enhanced distance measurement accuracy.
Interferometry is a well established metrology used extensively in microfabrication processes to measure and control a host of critical dimensions. It is especially important in manufacturing semiconductors and the like where requirements for precision are 10 to 40% of critical dimensions of 0.1 μm or less.
Integrated circuits made of semiconductor materials are constructed by successively depositing and patterning layers of different materials on a silicon wafer while it typically resides in a flat exposure plane having Cartesian x-y coordinates to which there is a normal z-direction. The patterning process consists of a combination of exposure and development of photoresist followed by etching and doping of the underlying layers followed by the deposition of subsequent layers. This process results in a complex and, on the scale of microns, very non-homogeneous material structure on the wafer surface.
Typically, each wafer contains multiple copies of the same pattern called “fields” arrayed on the wafer in a nominally rectilinear distribution known as the “grid.” Often, but not always, each field corresponds to a single “chip.”
The exposure process consists of projecting the image of the next layer pattern onto (and into) the photoresist that has been spun onto the wafer. For an integrated circuit to function properly, each successive projected image must be accurately matched to the patterns already on the wafer. The process of determining the position, orientation, and distortion of the patterns already on the wafer, and then placing them in the correct relation to the projected image, is termed “alignment.” The actual outcome, i.e., how accurately each successive patterned layer is matched to the previous layers, is termed “overlay.”
In general, the alignment process requires both translational and rotational positioning of the wafer and/or the projected image as well as some distortion of the image to match the actual shape of the patterns already present. The fact that the wafer and the image need to be positioned correctly to get one pattern on top of the other is obvious. Actual distortion of the image is often needed as well. Other effects, such as thermal and vibration, may require compensation as well.
The net consequence of all this is that the shape of the first-level pattern printed on the wafer is not ideal and all subsequent patterns must, to the extent possible, be adjusted to fit the overall shape of the first-level printed pattern. Different exposure tools have different capabilities to account for these effects, but, in general, the distortions or shape variations that can be accounted for include x and y magnification and skew. These distortions, when combined with translations and rotations, make up the complete set of transformations.
Since the problem is to successively match the projected image to the patterns already on the wafer, and not simply to position the wafer itself, the exposure tool must effectively be able to detect or infer the relative position, orientation, and distortion of both the wafer patterns themselves and the projected image.
It is difficult to directly sense circuit patterns themselves, and therefore, alignment is accomplished by adding fiducial marks or “alignment marks” to the circuit patterns. These alignment marks can be used to determine the reticle position, orientation, and distortion and/or the projected image position, orientation, and distortion. They can also be printed on the wafer along with the circuit pattern and hence can be used to determine the wafer pattern position, orientation, and distortion.
Alignment marks generally consist of one or more clear or opaque lines on the reticle, which then become “trenches” or “mesas” when printed on the wafer. But more complex structures such as gratings, which are simply periodic arrays of trenches and/or mesas, and checkerboard patterns are also used. Alignment marks are usually located either along the edges of “kerf” of each field or a few “master marks” are distributed across the wafer. Although alignment marks are necessary, they are not part of the chip circuitry and therefore, from the chip maker's point of view, they waste valuable wafer area or “real estate.” This drives alignment marks to be as small as possible, and they are often less than a few hundred microns on a side.
Alignment sensors are incorporated into the exposure tool to “see” alignment marks. Generally, there are separate sensors for the wafer, the reticle, and/or the projected image itself. Depending on the overall alignment strategy, these sensors may be entirely separate systems or they may be effectively combined into a single sensor. For example, a sensor that can see the projected image directly would nominally be “blind” with respect to wafer marks and hence a separate wafer sensor is required. But a sensor that “looks” at the wafer through the reticle alignment marks themselves is essentially performing reticle and wafer alignment simultaneously and hence no separate reticle sensor is necessary. Note, that in this case, the positions of the alignment marks in the projected image are being inferred from the positions of the reticle alignment marks, and a careful calibration of reticle to image positions must have been performed before the alignment step.
Furthermore, as implied above, essentially all exposure tools use sensors that detect the wafer alignment marks optically. That is, the sensors project light at one or more wavelengths onto the wafer and detect the scattering/diffraction from the alignment marks as a function of position in the wafer plane. Many types of alignment sensors are in common use and their optical configurations cover the full spectrum from simple microscopes to heterodyne grating interferometers. Also, since different sensor configurations operate better or worse on given wafer types, most exposure tools carry more than one sensor configuration to allow for good overlay on the widest possible range of wafer types.
The overall job of an alignment sensor is to determine the position of each of a given subset of all the alignment marks on a wafer in a coordinate system fixed with respect to the exposure tool. These position data are then used in either of two generic ways termed “global” and “field-by-field” to perform alignment.
In global alignment, the marks in only a few fields are located by the alignment sensor(s), and the data are combined in a best-fit sense to determine the optimum alignment of all the fields on the wafer. In field-by-field alignment, the data collected from a single field are used to align only that field. Global alignment is usually both faster, because not all the fields on the wafer are located, and less sensitive to noise, because it combines all the data together to find a best overall fit. But, since the results of the best fit are used in a feed-forward or dead reckoning approach, it does rely on the overall optomechanical stability of the exposure tool.
Alignment is generally implemented as a two-step process; that is, a fine alignment step with an accuracy of tens of nanometers follows an initial coarse alignment step with an accuracy of micrometers, and alignment requires positioning the wafer in all six degrees of freedom: three translation and three rotation. But adjusting the wafer so that it lies in the projected image plane, i.e., leveling the wafer and focusing the projected image, which involves one translational degree of freedom (motion along the optic axis, the z-axis or a parallel normal to the x-y wafer orientation) and two rotational degrees of freedom (orienting the plane of the wafer to be parallel to the projected image plane), is generally considered separate from alignment.
Only in-plane translation (two degrees of freedom) and rotation about the projection optic axis (one degree of freedom) are commonly meant when referring to alignment. The reason for this separation in nomenclature is the difference in accuracy required. The accuracy required for in-plane translation and rotation generally needs to be on the order of several tens of nanometers to on the order of nanometers or about 20 to 30% of the minimum feature size or critical dimension (CD) to be printed on the wafer. Current state-of-the-art CD values are on the order of 100 nm, and thus, the required alignment accuracy is significantly less than 100 nm. On the other hand, the accuracy required for out-of-plane translation and rotation is related to the total usable depth of focus of the exposure tool, which is generally closer to the CD value. Thus, out-of-plane focusing and leveling the wafer require less accuracy than in-plane alignment. Also, the sensors for focusing and leveling are usually completely separate from the “alignment sensors”, and focusing and leveling do not usually rely on patterns on the wafer. Only the wafer surface or its surrogate needs to be sensed. Nevertheless, this is still a substantial task requiring, among other things, precise knowledge about the vertical position (the altitude) of the optical projection system above the wafer.
To achieve alignment, it is known to use plane mirror interferometers, passive zero shear interferometers, and interferometers having an active beam steering element, i.e., active zero shear interferometers. In active and passive zero shear interferometers, the precision of distance measurements is enhanced through the use of beam conditioning to assure that beams carrying distance information are appropriately aligned to provide optimal signal. With respect to the passive zero shear interferometers, a plane mirror measurement object is used as an element in the beam conditioner. For such interferometers, see, for example, U.S. Provisional Patent Application Nos. 60/314,345 entitled “Passive Zero Shear Interferometers Using Angle Sensitive Beam-Splitters” filed on Aug. 23, 2001, now U.S. patent application Ser. No. 10/207,314 entitled “Passive Zero Shear Interferometers” filed on Jul. 29, 2002; 60/314,568 entitled “Zero Shear Plane Mirror Interferometer” filed on Aug. 23, 2001, now U.S. patent application Ser. No. 10/227,167 entitled “Multiple-Pass Interferometry” filed on Aug. 23, 2002; and 60/314,569 entitled “Zero Shear Non-Plane Mirror Interferometer” filed on Aug. 23, 2001, now U.S. patent application Ser. No. 10/227,166 entitled “Optical Interferometry” filed on Aug. 23, 2002;; and 60/352,425 entitled “Reduced Differential Beam Shear Multiple-Degrees Of Freedom Interferometers” filed on Jan. 28, 2002, now U.S. patent application Ser. No. 10/352,616 entitled “Multiple-Pass Interferometry” filed on Jan. 28, 2003.
In active zero shear interferometers, dynamic elements are used in the beam conditioner wherein the angular orientation of the dynamic element is controlled via feedback and/or feed forward arrangements to assure that beams carrying distance information are appropriately aligned to provide optimal signal. Such interferometers are shown, for example, in commonly owned International Application No. PCT/US00/12097 filed May 5, 2000 and entitled “Interferometry Systems Having a Dynamic Beam-Steering Assembly For Measuring Angle and Distance” and published on Nov. 19, 2000 as WO 00/66969 and in commonly owned U.S. Provisional Applications 60/314,570 filed Aug. 23, 2001 and entitled “Dynamic Interferometer Controlling Direction Of Input Beam”, now U.S. patent application Ser. No. 10/226,591 filed on Aug. 23, 2001 and published on Mar. 6, 2003 as US-2003-0043384 and 60/356,393 filed Feb. 12, 2002 and entitled “Interferometer With Dynamic Beam Steering Element Redirecting Input Measurement Beam Component And Output Reference Beam Component”, now U.S. patent application Ser. No. 10/364,666 filed on Feb. 11, 2003 and entitled “Interferometer With Dynamic Beam Steering Element”, wherein all three applications are in the name of Henry A. Hill.
However, even with passive zero shear and active zero shear interferometers, the shape of various reflecting elements impacts on the achievable precision in distance and angle measurements as stage mirrors undergo their various motions. The shape of the various reflecting elements impacts on the achievable precision because slope changes of reflecting elements in optical paths influence optical path lengths and beam directions. Typically, the shape of such reflecting elements, such as thin high aspect ratio mirrors, is characterized off-stage and the reflecting elements are then mounted on-stage. However, this is often unacceptable because precision of off-stage characterization is not sufficient, and/or the mounting process itself distorts the shape of the element compared with its inspected shape, and this change in shape can introduce measurement errors.
Accordingly, it is an object of the present invention to provide interferometric apparatus and methods by which maps of the surface topography of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage with high spatial resolution to develop correction signals or check precision of correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces wherein the interferometric apparatus may comprise integral optical assemblies.
It is another object of the present invention to provide interferometric apparatus and methods by which maps of the surface topography of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage with high spatial resolution to develop correction signals or check precision of correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces wherein only scanning in one or two orthogonal axes is required wherein the interferometric apparatus may comprise integral optical assemblies.
It is another object of the present invention to provide interferometric apparatus and methods by which maps of the surface topography of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage with high spatial resolution to develop correction signals or check precision of correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces arranged in orthogonal planes wherein the interferometric apparatus may comprise integral optical assemblies.
It is another object of the present invention to provide interferometric apparatus and methods by which maps of the surface topography of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage with high spatial resolution to develop correction signals or check precision of correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces arranged in orthogonal planes wherein only scanning in one or two orthogonal axes is required and wherein the interferometric apparatus may comprise integral optical assemblies.
It is yet another object of the present invention to exploit information generated from the operating properties of plane mirror interferometers, passive zero shear interferometers, and active zero shear interferometers by which the shapes of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage to develop a high spatial resolution map of the reflecting surfaces represented by datum lines that can be subsequently used to generate correction signals for compensation of errors in both optical path lengths and beam directions related to shapes of the reflecting surfaces wherein the interferometers may comprise integral optical assemblies.
It is yet another object of the present invention to exploit information generated from the operating properties of plane mirror interferometers, passive zero shear interferometers, and active zero shear interferometers by which the shapes of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage to develop a high spatial resolution map of the reflecting surfaces represented by datum lines that can be subsequently used to generate correction signals for compensation of errors in both optical path lengths and beam directions related to shapes of the reflecting surfaces wherein only scanning in one or two orthogonal axes is required and wherein the interferometers may comprise integral optical assemblies.
It is yet another object of the present invention to exploit information generated from the operating properties of plane mirror interferometers, passive zero shear interferometers, and active zero shear interferometers by which the shapes of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage to develop high spatial resolution maps of the reflecting surfaces comprising datum lines on the surfaces of the reflecting surfaces and local rotations of the surfaces about the datum lines that can be subsequently used to generate correction signals for compensation of errors in both optical path lengths and beam directions related to shapes of the reflecting surfaces wherein the interferometers may comprise integral optical assemblies.
It is yet another object of the present invention to exploit information generated from the operating properties of plane mirror interferometers, passive zero shear interferometers, and active zero shear interferometers by which the shapes of on-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage to develop high spatial resolution maps of the reflecting surfaces comprising datum lines on the surfaces of the reflecting surfaces and local rotations of the surfaces about the datum lines that can be subsequently used to generate correction signals for compensation of errors in both optical path lengths and beam directions related to shapes of the reflecting surfaces wherein only scanning in one or two orthogonal axes is required and wherein the interferometers may comprise integral optical assemblies.
It is yet another object of the present invention to provide interferometric apparatus and methods by which the shapes of off-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off-stage to develop with high spatial resolution correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces wherein the interferometric apparatus may comprise integral optical assemblies.
It is yet another object of the present invention to provide interferometric apparatus and methods by which the shapes of off-stage reflecting elements, such as thin high aspect ratio mirrors, may be measured in situ as well as off stage to develop with high spatial resolution correction signals that compensate for errors in both optical path lengths and beam directions related to shapes of reflecting surfaces wherein only scanning in one or two orthogonal axes is required.
Other objects of the present invention will, in part, be obvious and will, in part, appear hereinafter when reading the following detailed description in conjunction with the drawings.